1. Field of the Invention
The present invention relates to a biaxially oriented film which is hardly scratched at surface and which is excellent in abrasion resistance and scratch resistance of magnetic surface at a high running speed when it is processed into a magnetic recording medium.
2. Description of the Prior Art
Generally, a thermoplastic polyester, for example, polyethylene terephthalate, has excellent mechanical properties and chemical properties, and is broadly used for a formed product such as a film or a fiber. When a thermoplastic polyester is processed into and used as a formed product, the slipping property and the abrasion resistance thereof greatly affect the working ability in the production process or in the processing processes for various uses. For example, when the polyester is used as a base film of a magnetic tape, if these properties are insufficient, the friction between a coating roll and the film in the production process for the magnetic tape becomes large, wrinkles and scratches are likely to occur in the film. Further, the powders abraded from the film are liable to occur, and missing of coating occurs in the process for coating a magnetic layer, and as a result, missing of magnetic record (drop out) is likely to occur. Further, recently a problem has been pointed out that a magnetic surface is scratched by contact of the magnetic surface and a non-magnetic surface in a winding process after coating of a magnetic material and calendering and the scratches cause reduction of image quality such as drop out when used as a product.
As a conventional method for improving the slipping property of a film, a method for containing inorganic particles such as titanium dioxide, calcium carbonate or silicon dioxide particles in the polyester is proposed in many disclosures. However, since these inorganic particles are hard and poor in the affinity with polyester, for example, when the polyester is used as a film and an external force caused by a calendering in a magnetic layer coating process, a contact with a roll during running of the film or a contact between the surfaces of the film in winding is applied to the film, the particles easily fall off from the film and this causes occurrence of chipped substances, deterioration of the slipping property and occurrence of wrinkles on the film surface. Moreover, because the fallen particles themselves are hard, the chipped substances and the wrinkles on the film surface increase multiplicatively as time passes. Thus, when the amount of the chipped substances increases and the wrinkles occur on the film surface, missing of coating in a magnetic layer coating process and drop out are liable to occur. Further, a dirt of a calender roll in the process for coating a magnetic layer remarkably deteriorates the working ability in the production of a magnetic recording film. Thus, such scratches and poor abrasion resistance caused by the fact that particles such as inorganic particles, which are hard and poor in the affinity with polyester, are likely to directly receive an external force, have been desired to be improved.
As a method for improving the affinity of particles with polyester, a method for treating the surfaces of inorganic particles or a method for using organic particles are proposed. As the surface treatment of inorganic particles, for example, the surface treatment by polyacrylic acid polymers is proposed in JP-A-SHO 63-128031, the surface treatment by a phosphorus compound is proposed in JP-A-SHO 62-235353 and 63-234039, the surface treatment by a coupling agent is proposed in JP-A-SHO 62-223239 and 63-312345, the surface treatment by a silane compound is proposed in JP-A-SHO 63-304038, and the surface treatment by the graft due to glycol is proposed in JP-A-SHO 63-280763, respectively. However, it is difficult to stand both of an appropriate hardness and a good affinity of the particles, and a sufficient abrasion resistance cannot be obtained by these methods. As the organic particles, crosslinked polymeric particles are proposed in, for example, JP-B-SHO 63-45409, JP-A-SHO 59-217755 and JP-A-HEI 2-189359, respectively. However, the particles proposed by these methods do not have specified strength and degree of crosslinkage, and even if these methods are employed, a sufficient abrasion resistance at a high running speed cannot be obtained.
As a biaxially oriented thermoplastic resin film used for magnetic recording media, a laminated film, wherein a layer containing inorganic or organic particles is laminated on at least one surface of a film and the relationship between the diameter of the particles and the thickness of the laminated layer is specified, thereby improving the handling ability, the running ability and the abrasion resistance, is known (JP-A-HEI-2-77431). In such a film, because protrusions having a uniform height are formed on the film surface, it is possible to remarkably decrease the damage to a magnetic surface such as the transfer of the shape of excessive large protrusions to the magnetic surface in curing process or the thrust up due to the protrusions in calendering process. Particularly, such a film is suitable to a base film for a metal tape such as an 8 mm VTR tape or a business-use VTR tape.
Recently in magnetic tape makers, however, further cost down is planned, and a higher production speed or in a part of metal tapes, omission of a backcoat, which has been provided on a running surface of a conventional tape, is investigated. In a case where a conventional base film for a metal tape is made as a backcoatless film, when the fast feed/rewind (FF/REW) operation is repeatedly carried out, there occurs a problem that the running surface of the film is scratched or that the drop out increases by generation of chipped powder. In particular, because the speed of the FF/REW is expected to be increased in the future, it will become extremely important to further increase the running durability of the surface of a base film.